In a current mode converter, typically, the output voltage is generated by alternatively switching a pair of high side and low side switches coupled between an input voltage and ground to produce an output current to charge an output capacitor. To reduce the influence to load transient resulted from the equivalent series resistance (ESR) of the output capacitor, voltage droop function is generally employed. To illustrate the voltage droop tuning, FIG. 1 schematically shows the waveforms of the output voltages of a current mode converter with and without voltage droop function in a load transient, in which waveform 100 represents the output voltage of a current mode converter without voltage droop function in a load transient, and waveform 102 represents the output voltage of a current mode converter having voltage droop function in a load transient. As shown at time T1, when load changes from light to heavy, the output voltage of a converter without voltage droop function instantly drops ΔV and then recovers to the original level, as depicted by portion 104, while the output voltage of a converter having voltage droop function instantly drops ΔV and then maintains at the lower level, as depicted by portion 106. Until the load changes from heavy back to light at time T2, the output voltage of the converter without voltage droop function instantly jumps ΔV and then recovers to the original level, as depicted by portion 108, while the output voltage of the converter having voltage droop function recovers from the lower level to the original level, as depicted by portion 110. In this figure, Vmax denotes the maximum voltage the converter could generate, and Vmin denotes the minimum voltage the converter could generate, and during load transient, i.e., from time T1 to T2, ΔVC,ESR,1 is the tolerance for the output voltage of the converter without voltage droop function available for the influence resulted from the equivalent series resistance of the output capacitor, and ΔVC,ESR,2 is the tolerance for the output voltage of the converter having voltage droop function available for the influence resulted from the equivalent series resistance of the output capacitor. Since ΔVC,ESR,1 is smaller than ΔVC,ESR,2, a converter without voltage droop function needs more output capacitors to reduce the influence to the output voltage resulted from the equivalent series resistance of the output capacitor than a converter having voltage droop function. Accordingly, converter having voltage droop function is superior in cost.
Moreover, for faster response to load transient, a conventional current mode converter is operated with varying frequency implemented with hysteretic control. Unfortunately, the control for such operation with varying frequency is much more complicated than that of fixed-frequency operation. On the other hand, in a multi-phase converter, for balancing the inductor currents between all phases, all the inductor currents in the phases are summed and averaged for control, thereby requesting additional circuitry for the summation and averaging operations. As a result, the complexity and cost of the system increase.
Accordingly, it is desired a fixed-frequency current mode converter, which has voltage droop function and faster response to load transient, and achieves inductor current balance between all phases without additional circuitry.